This invention relates to methods for bonding two semiconductor structures together in order to make structures for microelectronics, optics or optoelectronics. In particular, the invention relates to preparing bonding surfaces of the structures.
Wafer surfaces have to be cleaned before bonding in order to obtain good bonding results. For example, it is known to subject wafer surfaces to one or more chemical treatments, such as an RCA treatment, to remove isolated particles and metallic and organic contaminants. Another known chemical surface treatment utilizes dry ozone to clean, wherein the dry ozone is obtained by transforming atmospheric oxygen into ozone under the action of TV rays. The dry ozone reacts with hydrocarbon surface contaminants and eliminates them by creating volatile species.
Other techniques for preparing wafer surfaces for bonding are of the mechanical type. Friction forces can be applied to the surface of the wafers to remove isolated particles. For example, lightweight polishing or brushing may be used.
If there is no particulate, no organic residue and no surface defects, the bonding quality between wafers depends on the adhesive technique used to bond the wafers. For example, two wafers can be bonded together by molecular bonding, at least initially. Specific preliminary treatments can advantageously be used to improve the molecular bonding (and therefore the bonding quality) between two wafers. The improvement in the bond usually corresponds to an increase in the mechanical strength and/or an increase in the quality of the bonding interface. Thus, an increase in the mechanical strength of the bond between the wafers can be achieved if the hydrophilic properties of the surfaces to be bonded are improved (to learn more, one can refer to, for example, the document entitled “Influence of Surface Characteristics on Direct Wafer Bonding” by O. Rayssac et al. “Proceedings of the 2nd International Conference on Materials for Microelectronics”, IOM Communications, p. 183, 1998).
For example, it is known to prepare wafers before bonding by using chemical solutions, such as a solution containing ozone or a solution containing a mix of sulphuric acid and oxygenated water (also called an SPM—“Sulfuric Peroxide Mixture”) solution. Use of such solutions facilitate the removal of hydrocarbon contaminants and also improves the wettability of the surface of the wafers by improving the hydrophilic properties, and therefore increases the bonding energy.
In general, RCA type solutions are used for cleaning purposes, and to increase the hydrophilic properties of surfaces to be bonded by molecular bonding. After a chemical treatment, at least one of the surfaces to be bonded may also be exposed to a plasma treatment in order to increase the forces that will bind the two wafers together when they are brought into contact. The surfaces may be brought into contact and possibly accompanied or followed by a heat treatment to reinforce the bonding. Further information may be found in the article, for example, by T. Suni et al., Journal of Electromechanical Society 149, G348, 2002 “Effects of plasma activation on hydrophilic bonding of Si and SiO2”.
A mechanical chemical polishing process, such as Chemical Mechanical Planarization (CMP), may be used to activate bonding surfaces if it is used in a specific chemical environment. Thus, for example, a CMP in a basic medium on silicon or silicon-oxide surfaces can activate such surfaces to reinforce their molecular bonding characteristics. Such operation is described in an article by R. Stengl, T. Tang and U. Gosële entitled “A Model for the Silicon Wafer Bonding Process” (Japanese Journal of Applied Physics, 28, p. 1735, 1989). A brushing step may then optionally be applied in order to eliminate particles resulting from the polishing. However, even slight polishing is abrasive, and this can thus affect the quality of the treated material. Thus, the previously discussed treatments are not always sufficient. In particular, such treatments are insufficient if followed by other treatments that reduce the surface quality and/or activate properties that adversely affect the surface quality, or if a long duration separates the treatments from the bonding step, because a waiting period allows contaminants and gaseous residues in the atmosphere to adhere to the surface.